This invention relates to a hinge structure with torsion bars and more particularly to a hinge structure for a device which is separable into an upper unit and a lower unit in such a way that a biasing force is applied to the upper unit in the direction of opening it with respect to the lower unit.
Many devices containing component parts which require maintenance work have a housing which is separable into an upper unit and a lower unit such that the upper unit can be opened and closed with respect to the lower unit around an axis at one of its edges. An electrophotographic copying machine with a housing of this type is described in U.S. Pat. No. 4,416,536, issued Nov. 22, 1983 and assigned to the present assignee (and incorporated herein by reference). Since the upper unit of such a housing frequently contains many parts, its weight can be considerable. In order to prevent such an upper unit from dropping down by its own weight while an operator is engaged in a maintenance work, the housing is frequently provided with means for applying elastic biasing forces which tend to keep the upper unit in its opened position. For improving the stability of motion for the upper unit, such biasing forces are usually applied near the ends in the axial direction of the opening and closing motion. Hinge structures with torsion bars as described, for example, in U.S. patent application Ser. No. 785,216 assigned to the present assignee (and also herein incorporated by reference) are sometimes used for this purpose because of their advantageously simple structure and low cost. The hinge structure includes two elongated pieces of elastic material (torsion bars) such that identical elastic forces are applied at the both end sections in the axial direction of the opening and closing motion. If the weight distribution of the upper unit is not uniform because of the positioning of the component parts contained therein, however, there appears a difference in the load on each of the torsion bars. Since the two torsion bars are intended to provide identical forces, a difference in load results in a shearing stress on the upper unit. In order to protect the unit against such a shearing stress, its rigidity must be increased and this means an increase in cost.
It may be considered to use compressive springs for the same purpose such that the elastic forces can be easily varied, depending on the load to be supported. If the weight distribution of the upper unit is not uniform, however, springs of different sizes must be provided on both end sections in order to eliminate the shearing stress on the upper unit. This results in an increased number of component parts. Moreover, changes in the elastic force of the springs necessitate readjustments of component parts and this, too, has the effect of increased cost.